In general, hammer bit equipment used in drilling a borehole includes a rotation apparatus, a striking apparatus and a drilling apparatus. The present invention is directed to the drilling apparatus located at the lowest portion of the hammer bit equipments. The drilling apparatus can be divided into an eccentric type, an extending type and a blade extending type depending on means for extending the diameter of the borehole, for instance, the structure of a reamer, extending blades or arms.
As disclosed in U.S. Pat. No. 4,770,259 (Published on Sep. 13, 1988), the eccentric type drilling apparatus includes a drill string and a cutting device connected to a lower end portion of the drill string. Further, the cutting device consists of an intermediate portion rotating within the drill string, and an outer surface. Also, a reamer is installed in the intermediate portion so that it is offset with regard to the center axis. Accordingly, the reamer extends the diameter of the borehole by eccentric rotation in the eccentric type drilling apparatus.
In addition, as shown in Japanese Patent Laid-Open Publication No. 2710192 (published on Nov. 29, 1994), the extending type drilling apparatus comprises a device driven by means of an air pump, a bit device installed at a distal end of the device, and an extending blade installed between the device and the bit device. The extending blade is secured at an upper end to the device by means of a pin so that it can move angularly in the vertical direction, and the upper end portion of the bit device is formed to have an inclined surface. Therefore, the lower end portion of the extending blade is configured to diverge along the inclined surface of the bit device when the device and the bit device come close to each other.
Further, as shown in U.S. Pat. No. 5,787,999 (issued on Aug. 4, 1998), the blade extending type drilling apparatus includes a driver, under-reamer arms, and a pilot bit, in which a plurality of arms are constructed to project and retract while rotating from the center of the pilot bit to the inclined direction by means of a rotation force of the driver.
However, with regard to the eccentric type drilling apparatus, it is impossible to carry out rapid drilling work as the reamer rotates eccentrically, and there is a problem in that connection portion of the intermediate portion is liable to be damaged easily under high load. Also, with regard to the extending type drilling apparatus, it is impossible to use it under high load as the extending blade is extended with its angular movement to the longitudinal direction, and the fixing pin is apt to be damaged easily.
Further, in contrast to the eccentric or extending type drilling apparatus, although the blade extending type drilling apparatus can be used for high load drilling apparatus, there is a problem in that the contact portion between the arms and the pilot bit is susceptible to serious abrasion because the rotation force of the driver should spread the arms forcibly and rotate even the pilot bit via the arms at the same time, and the securing pin for fixing the arms are damaged frequently. Additionally, when the arms return to their original positions after the completion of the drilling work, sludge is liable to be jammed in the space where the arms return to thereby hinder the arms' return to their original positions, so that it becomes somewhat difficult to retract the drilling apparatus from the casing to fit into the borehole.